Accommodation Reflex
What is the accommodation Reflex?
The accommodation reflex is an automatic coordinated change that occurs in the eyes when you switch focus from an object that’s far away to one that’s closer.
The reflex involves three responses:
- convergence of the eyes
- pupillary constriction
- lens accommodation
How do the eyes change during accommodation?
Convergence of the eyes
Convergence is the simultaneous movement of both eyes inward when viewing a near object.
Convergence of the eyes keeps the image of the object centered on the fovea, the part of the retina where visual acuity is highest.
Hold a pencil in front of your eyes with an outstretched arm. Focus on the pencil and slowly move it in towards your nose.
Your eyes will begin to move inwards (convergence) to keep the pencil in focus. You will notice that anything in the background in the distance will become blurred.
Now, with the pencil held closer to your nose, bring your focus to the wall or item in the distance. Your eyes will no longer be working together to focus on the pencil and will result in double vision.
Pupillary Constriction
When viewing objects up close, the sphincter muscles of the iris constrict the pupil to restrict the amount of light that enters the eye.
The constricted pupil prevents diverging light rays from hitting the periphery of the retina and causing a blurred image. When the pupil constricts it allows light to focus on the fovea of the retina for clear vision.
For example, when using a camera, a smaller aperture (opening, hole) gives a greater depth of focus and reduces image distortion.
Lens Accommodation
The lens changes shape to enable the eye to focus at various distances.
The ciliary muscles control the convexity (outward curvature) of the lens in response to the distance of the object being viewed.
When viewing near objects, the lens is rounder in shape which increases its power to bend light onto the fovea.
Why is the accommodation reflex important?
The accommodation reflex allows us to clearly focus on objects up close when switching from viewing things at a distance. All three responses must occur to see items clearly.
If the eyes do not converge appropriately, double vision will occur.
If the pupils remain dilated, light will not focus on the fovea and the image will be blurred.
If the lens does not change shape the eyes would not be able to focus on items at various distances. The flattening and rounding of the lens changes the refractive power in order to appropriately focus light onto the retina for clear vision.
